62 PHYSIOGRAPHY. [chap. 



six-pointed stars variously modified. Each star has an icy 

 centre as a nucleus, from which six little spicules or rods 

 of ice are shot forth at regular angles ; and from the sides 

 of these rays, secondary rays, or raylets, may be given off 

 at the same angle, thus producing complex stars of great 

 beauty, but, in spite of their complexity, always true to the 

 hexagonal symmetry of the system to which ice belongs. 

 Each part of the pattern is repeated round the centre six 

 times, as is generally the case with the beautifully-sym- 

 metrical shapes seen in a common kaleidoscope. 



Although ice does not ordinarily exhibit well-defined 

 crystals, it is nevertheless built up of crystalline particles 

 interlaced together. Prof Tyndall has shown us how to 

 reveal this beautiful architecture, by submitting a block 

 of ice to the action of a sunbeam, or even to a beam of 

 electric light. Part of the heat enters the solid, and pro- 

 duces internal liquefaction, which proceeds with great 

 regularity. Small shining points first appear in the ice, 

 and around each of these points, as a centre, siji rays 

 shoot forth, producing figures such as those represented 

 in Fig. 17.1 These beautiful forms, which commonly re- 

 semble blossoms with six petals or floral leaves, are not 

 solid crystals, like our crystals of snow, but are simply 

 hollow spaces of regular shape filled with water; they 

 may indeed be called " negative " or " inverse " crystals, 

 developed by the breaking-down or " decrystallisation " 

 of the ice. The ice is in fact crystalline, whilst the snow 

 is crystallised. 



When there is much wind astir, the snow falls in 

 shapeless masses, or even in small hardened pellets. If 

 the snow-flakes become partiall)'^ melted in their descent 

 by falling through a layer of warm moist air, they produce 



1 See Tyiidall's Heat as a Made 0/ Motion, 5th ed. p. Ii. 



